1. Field of the Invention
The present invention relates to communication equipment and, more specifically, to equipment for wireless local area networks (WLANs).
2. Description of the Related Art
IEEE Standard 802.11 has emerged as a prevailing technology for broadband access in WLAN systems and is regarded by many as a wireless version of Ethernet. The 802.11 legacy medium access control (MAC) is based upon best-effort service, which does not support quality of service (QoS) for users or applications. However, the increasing demand for streaming media (e.g., voice and video) transmissions, which typically have certain QoS requirements, prompted the 802.11 Working Group to begin work on an extension to the standard. The upcoming 802.11e supplement standard will define enhancements to the legacy 802.11 MAC providing a QoS support facility.
Draft standard 802.11e (version D4.0 of November, 2002), the teachings of which are incorporated herein by reference, provides two mechanisms for the support of applications with QoS requirements. The first mechanism, designated as Enhanced Distributed Coordination Function (EDCF), is based on differentiating priorities at which data traffic is to be delivered. The second mechanism allows for scheduling of transmission opportunities with the hybrid coordinator located at the access point (AP) of the WLAN. Various time-sharing schemes between the two mechanisms are also possible.
The EDCF is based on eight priority values, designated 0 to 7, which are analogous to IEEE 802.1d priority tags. Each data packet to be transmitted is provided with a priority value, based on which the packet is sorted into one of the following four access categories (ACs): best effort, video probe, video, and voice. Each AC contends for access to the wireless medium based on the corresponding QoS parameter set (PS) including, for example, the values of (i) CWmin and CWmax specifying the contention window and (ii) arbitration inter-packet spacing. After winning a contention, an AC is allowed to transmit one or more packets in addition to the first packet without having to re-contend for access to the wireless medium for the transmission of the additional packets. Such transmission of multiple packets is referred to as data bursting.
FIG. 1 shows a block diagram of a representative WLAN system 100 having two prior-art 802.11e-compliant stations (STAs) 102 and 112 communicating over a wireless communication channel 122. For illustration purposes, STA 102 is a transmitting station and STA 112 is a receiving station. Four data streams S0-S3 corresponding to four access categories AC-n, where n=0, 1, 2, 3, are applied to STA 102. For each AC, STA 102 has a corresponding buffer 104-n configured to queue data to be transmitted over channel 122 using a transmitter 108. A controller 106 of STA 102 implements 802.11e MAC functions to govern application of data from buffers 104-n to transmitter 108. More specifically, controller 106 is adapted to treat each AC-n as a separate EDCF contending entity having its own QoS parameter set (PS[n]) and maintaining its own back-off counter (BC[n]). When more than one AC-n of STA 102 finish their back-off procedures at the same time, controller 106 resolves any collision between those ACs using a virtual collision handler (VCH) in favor of the AC having the highest priority. For example, if the collision occurs between the video and best-effort ACs, the video AC wins the contention and is given access to transmitter 108. At STA 112, a receiver 118 receives transmissions from STA 102 and a processor 116 processes the output of receiver 118 to generate four data streams S0′-S3′ corresponding to data streams S0-S3.
One problem with system 100 is that transmission of a given media stream, e.g., stream S0, is relatively difficult to adapt to the fluctuating conditions of channel 122. For example, a representative prior-art adaptation technique involves bit-rate adjustment at a source coder (not shown) that generates stream S0. This requires communication between the higher and lower network layers embodied by the source coder and processor 106, respectively, which increases the overall complexity of the WLAN system and adds latency to its operation. In the absence of such communication, STA 102 reacts to “bad-channel” conditions by dropping a certain number of data packets, which may result in unpredictable fluctuations of signal quality.